Why has my screen gone weird?

My s4 appears to be behaving normally except for the screen. When the screen is activated, it shows a steadily increasing array of mostly green, but also red and blue, dots. It doesn’t show anything else at all. When this first happened yesterday, I also noticed that the phone had a bulge in the middle of it - the battery had swollen up. So I removed the battery, left the phone overnight, and the next day when I put the battery back in and turned the phone on it was fine. SO, I just installed a NEW battery, but after working fine for about 10 minutes, the screen is doing the same thing as before.

I have tried opening up the phone and the bit that connects the screen to the motherboard seems to be clipped in fine.

Has this happened to anybody before? If I intall a new screen will it work? Or is something else causing the problem?